分光光度法测定水中游离氯和总氯曲线及加标回收问题探讨

Discussion on the Problems of Curve Drawing and Standard Recovery in the Determination of Free Chlorine and Total Chlorine in Water by Spectrophotometry

为了解决分光光度法(HJ586-2010)在曲线绘制时存在低浓度点不显色现象,用于加标的次氯酸钠标准液不稳定且操作繁琐的现实缺陷。采用不同显色时间及取消曲线制作过程中在加入硫酸溶液前先加入约50 mL水的实验环节,并采用碘酸钾代替氯制剂进行加标回收。实验结果表明,随着显色时间的延长,曲线斜率逐渐降低,曲线的第一点吸光值始终偏低,无法取得理想的标准曲线,在取消加入约50 mL水的环节后,无论是高浓度曲线还是低浓度曲线的截距、斜率和相关系数均符合污水监测技术规范要求;碘酸钾标准溶液按照绘制标准曲线的方法加入硫酸和氢氧化钠溶液处理好以后再进行水样加标,获得的加标回收率,符合《环境水质监测质量保证手册》的要求。分析实验结果可知,显色时间不是标准曲线绘制成败的决定因素;碘酸钾和碘化钾反应时溶液的酸度高低决定了碘分子或[I_(3)]^(-)在规定时间内生成是否完全,从而决定了标准曲线绘制的成功与否;用碘酸钾溶液代替次氯酸钠溶液进行加标回收,可以解决次氯酸钠标准溶液操作繁琐且不稳定的难题。

In order to solve the problem that the low concentration points do not show color in curve drawing by spectrophotometric method(HJ586-2010), and sodium hypochlorite standard liquid used for standard recovering is unstable and the operation is complicated. In this study, different color rendering time was used, the experimental step of adding about 50 mL water before adding sulfuric acid solution during curve making process was canceled, and potassium iodate was used instead of chlorine preparation for standard recovering. The experimental results showed that, the curve slope decreased gradually with the extension of color rendering time, the light absorption value of the first point of the curve was always low, so the ideal standard curve cannot be obtained, after canceled the experimental step of adding about 50 mL water, the intercept, slope and correlation coefficient of both the high concentration curve and the low concentration curve all met the requirements of sewage monitoring technical specifications. The potassium iodate standard solution was treated by adding sulfuric acid and sodium hydroxide solution according to the method of drawing standard curve, and then the standard recovery rate of water sample is in line with the requirements of "Environmental water quality Monitoring Quality Assurance Manual". According to the analysis of the experimental results, it is known that color rendering time is not the decisive factor for drawing standard curves;the acidity of the solution during the reaction of potassium iodate and potassium iodide determines whether the formation of iodine molecules or [I_(3)]^(-)is complete within the specified time, which determines the success of standard curve drawing. Using potassium iodate solution instead of sodium hypochlorite solution for standard recovery can solve the problem of complicated and unstable operation of sodium hypochlorite solution.